The present invention relates to a lithium secondary battery.
A lithium secondary battery is mainly composed of a positive electrode, a negative electrode, a non-aqueous electrolyte and a separator separating the positive electrode and the negative electrode from each other. The non-aqueous electrolyte is obtained by dissolving an electrolyte salt comprising an alkali metal salt such as LiPF6 in a non-aqueous solvent such as ethylene carbonate or dimethyl carbonate. The separator is insoluble in the above non-aqueous solvent, and is a porous film made of, for example, polyethylene or polypropylene resin.
A lithium secondary battery is a secondary battery having high energy density and capable of being miniaturized for weight-saving, and for attaining further higher performance, intensive investigation is now being made on the constitutive elements of the above battery.
For example, recently, lithium-containing transition metal oxides such as lithium cobaltate (LiCoO2) which show a high potential are used as active materials of positive electrodes, and carbon materials such as graphite are often used as negative electrode materials.
JP-A-4-319259, JP-A-4-319260, JP-A-5-6779, JP-A-5-6780 and JP-A-6-150928 disclose techniques according to which 1-10 mol % of lanthanum, zirconium, cobalt, yttrium or samarium based on cobalt of the active material of positive electrode is added and fired to cover the active material of positive electrode with an oxide of cobalt and the element added (for example, LaCoO3 in the case of using lanthanum), whereby the active material of positive electrode is stabilized to inhibit the decomposition of the electrolyte and improve the storage characteristics. Furthermore, JP-A-7-192721 discloses a technique according to which 0.1-20 mol % of a metal selected from the group of sodium, magnesium, aluminum, potassium, calcium and the like is added to the active material of positive electrode and is allowed to act as a catalyst poison for the decomposition reaction of the electrolyte, thereby to inhibit the decomposition of the electrolyte and improve the storage characteristics.
The inventors have made further various investigations in an attempt to provide batteries excellent in high rate characteristics and low-temperature characteristics. As one of the methods, it has been found that excellent characteristics can be obtained by adding to the active material of positive electrode a small amount of different elements of Groups IIA, IIIB, IVB, VB and VIB and lanthanide elements in the periodic table. It is considered that this is because addition of the above elements causes change in the surface state of the active material to increase the surface area. However, it has been seen that although the above characteristics are improved, amount of the gas evolved during storage at high temperatures increases. The cause therefor seems that the added elements together with the active material form an active site for evolution of gas on the positive electrode to cause decomposition of the electrolyte.
The above decomposition of the electrolyte is mainly due to the oxidative decomposition of the solvent on the positive electrode. Especially, when a cyclic carbonic acid ester or a cyclic carboxylic acid ester is used as a solvent, a ring opening reaction is apt to take place, and the ring opening reaction product readily undergoes oxidative decomposition on the positive electrode. Furthermore, when a non-cyclic carbonic acid ester is used as a solvent, an ester interchange reaction takes place. An intermediate product of this ester interchange reaction also readily undergoes oxidative decomposition on the positive electrode. These oxidative decomposition reactions involve evolution of gases such as carbon dioxide to result in reduction of battery voltage and deterioration of battery characteristics after storing of battery.
The inventors have conducted further research, and, as a result, have found that batteries excellent in storage characteristics at high temperatures as well as high rate characteristics and low-temperature characteristics can be provided by using an electrolyte having a specific composition in combination with the above-mentioned active material of positive electrode. Thus, the present invention has been accomplished.
That is, the present invention relates to a lithium secondary battery comprising a positive electrode having as an active material a lithium-containing composite transition metal oxide or a lithium-containing composite transition metal oxide in which a metal element other than the transition metal constituting the lithium-containing composite transition metal oxide is contained in the form of solid solution, the positive electrode further comprises one or more of metals of Groups IIA, IIIB, IVB, VB and VIB and lanthanide elements in the periodic table and compounds of these metals; a negative electrode; and a non-aqueous electrolyte containing a solvent and an electrolyte salt containing at least one member selected from the group of fluorine-containing inorganic anion lithium salts comprising LiPF6, LiBF4, LiAsF6 and LiSbF6 and at least one member selected from lithium imide salts represented by the following formula (1): 
(wherein R1 and R2 are independent of one another and represent CnX2n+1 or CnX2nxe2x88x921 in which n is an integer of from 1 to 8 and X is a hydrogen atom or a halogen atom).
The inventors consider that it is based on the following principle that evolution of gases can be inhibited by using an electrolyte containing the lithium imide salt.
A lithium imide salt such as LiN(CF3SO2)2 undergoes oxidative decomposition at a lower potential as compared with LiPF6. When a cyclic voltammetry is conducted using a platinum electrode as a working electrode, and lithium metal as a reference electrode and a counter electrode at room temperature, a current produced by the oxidative decomposition begins to flow at about 4.2 V (Li standard). In the case of usual lithium secondary batteries, since potential of the positive electrode reaches higher than 4.2 V at the time of full charging, the lithium imide salt added to the electrolyte is decomposed at the initial charging. At that time, the decomposition product covers the surface of the positive electrode to cover the active points which participate in the reaction of the electrolyte. As a result, evolution of gases at the time of charging can be inhibited. Moreover, the above decomposition product also covers the surface of the negative electrode and thus simultaneously inhibits the evolution of gases at the surface of the negative electrode. It further inhibits decomposition of the additives in the positive electrode and dissolution of them into the electrolyte.
The above storage characteristics depend on the amount of the lithium imide salt added, and as a result of intensive investigation conducted by the inventors, it has been found that the amount is suitably not less than 0.003 mol/l and not more than 0.50 mol/l. If the lithium imide salt is added in an amount of more than 0.50 mol/l, the quantity of electricity required for decomposition increases to cause increase of charging loss. If the lithium imide salt is added in an amount of less than 0.003 mol/l, the amount of the decomposition product of the imide salt is insufficient, and, hence, evolution of gases during storage at high temperatures cannot be sufficiently inhibited. The amount is more preferably not less than 0.003 mol/l and not more than 0.25 mol/l, most preferably not less than 0.003 mol/l and not more than 0.05 mol/l.
The technique of using a fluorine-containing inorganic anion lithium salt and a lithium imide salt in admixture is disclosed, for example, in JP-A-10-189045. On the other hand, the present invention has been made to solve the problem peculiar when one or more of metals of Groups IIA, IIIB, IVB, VB and VIB and lanthanide elements in the periodic table and compounds of these metals is added to the positive electrode, and the optimum amount of the imide salt in this case has been found.
By using the above-mentioned mixed electrolyte salts, a lithium secondary battery less in evolution of gases during storage at high temperatures and superior in reliability can be provided.